The present invention relates to a method to determine during a tracking mode a clock timing error used for synchronisation purposes in a multi-carrier transmission system an arrangement adapted to perform this method as and synchronisation units to be used in such a multi-carrier system.
Such a method and related equipment to perform this method are already known in the art, e.g. from the European Patent Application EP 0 453 203, entitled xe2x80x98Method and apparatus for correcting for clock and carrier frequency offset, and phase jitter in multicarrier modemsxe2x80x99 from applicant Telebit Corporation. Therein, phases of a few pilot carriers are detected and used to calculate a clock timing error named a phase correcting signal (see page 3, lines 11-35 of the cited European Patent Application). As is indicated on page 6, lines 24-26 of EP 0 453 203, the phase correcting signal is used in a phase-locked loop (PLL) to realise synchronisation between a transmitting multi-carrier modem and a receiving multi-carrier modem.
In case of a norrowband interferer in the vicinity of one of the pilot carriers whose phases are detected to calculate the clock timing error, use of phase information extracted from this pilot carrier renders the so called clock timing error or phase correcting signal less accurate as a measure for the timing difference between transmitting and receiving modem. As a consequence, synchronisation between the transmitting and receiving modem may be lost in the known system when one of the pilot carriers is affected by noise.
An object of the present invention is to provide a method, arrangement and synchronisation units similar to those known, but whose robustness for narrowband noise near the pilot carriers is optimised.
According to the invention, this object is achieved by a method to determine during a tracking mode in a multi-carrier system a clock timing error used for synchronisation purposes, the method comprising the steps of detecting phase errors for a plurality of pilot carriers and calculating the clock timing error from the phase errors, wherein a share of a phase error of the phase errors in the clock timing error depends on a value of a transmission quality parameter measured for a pilot carrier of the pilot carriers for whom the phase error is measured.
It is also achieved by an arrangement to determine during a tracking mode in a multi-carrier system a clock timing error used for synchronisation purposes, the arrangement comprising: phase error detection means whereto a multi-carrier signal is applied, the phase error detection means being adapted to detect phase errors for a plurality of pilot carriers; and calculation means, coupled to the phase error detection means, and adapted to calculate the clock timing error from the phase errors, wherein the arrangement further comprises: share determination means, having an output coupled to the calculation means and being adapted to receive via an input thereof values of a transmission quality parameter related to the plurality of pilot tones and to determine shares of the phase errors in the clock timing error from the values of the transmission quality parameter; and further in that the calculation means is adapted to receive via an input thereof the shares and to calculate the clock timing error from the shares and the phase errors.
It is further achieved by a synchronisation unit to be used in a multi-carrier system, the synchronisation unit comprising: skip and duplicate means, adapted to remove a sample from or to duplicate a sample in a multi-carrier signal when a clock timing error becomes larger than or equal to a sample period; phase rotation means, coupled in series with the skip and duplicate means, and adapted to apply a phase shift to each carrier in the multi-carrier signal proportional to the clock timing error and proportional to a frequency of the carrier; a clock timing error determination arrangement, coupled to the phase rotation means, and adapted to determine during a tracking mode of the multi-carrier system the clock timing error, the clock timing error determination arrangement comprising phase error detection means whereto a multi-carrier signal is applied, the phase error detection means being adapted to detect phase errors for a plurality of pilot carriers; and calculation means, coupled to the phase error detection means, and adapted to calculate the clock timing error from the phase errors; and a feedback loop coupled to the clock timing error determination arrangement and having an output coupled to inputs of both the skip and duplicate means and the phase rotation means, the feedback loop being adapted to feed back the clock timing error to both the skip and duplicate means and the phase rotation means, wherein the clock timing error determination arrangement further comprises share determination means, having an output coupled to the calculation means and being adapted to receive via an input thereof values of a transmission quality parameter related to the plurality of pilot tones and to determine shares of the phase errors, in the clock timing error from the values of the transmission quality parameter; and further in that the calculation means is adapted to. receive via an input thereof the shares and to calculate the clock timing error from the shares and the phase errors.
Indeed, giving the phase error detected for a first pilot carrier which is transferred with a low transmission quality a relatively low share in the clock timing error used for synchronisation, and giving the phase error detected for a second pilot carrier which is transferred with a high transmission quality a relatively high share in the clock timing error used for synchronisation, has a filtering effect on the norrowband noise which affects the transmission quality of the first pilot carrier for this clock timing error. As a consequence, the variance of the clock timing error is reduced according to the present invention resulting in a better tracking of the timing-locked loop whereto the clock timing error is applied as input. This implies that the synchronisation process is made less sensitive for narrowband noise.
It is to be noticed that the term xe2x80x98comprisingxe2x80x99, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression xe2x80x98a device comprising means A and Bxe2x80x99 should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Similarly, it is to be noticed that the term xe2x80x98coupledxe2x80x99, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression xe2x80x98a device A coupled to a device Bxe2x80x99 should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.
An additional feature of the present invention is such a method where the value of the transmission quality parameter is determined during an acquisition mode preceding the tracking mode.
Indeed, it can be expected that the transmission quality of the medium in between two multi-carrier modems at a certain frequency does not change abruptly, unless impulse noise disturbs the medium. If the transmission quality of the medium at that certain frequency is measured once during an acquisition mode or initialisation procedure, the measured quality can be used for a long period. In Discrete Multi Tone (DMT) systems such as an Asynchronous Digital Subscriber Line (ADSL) system, the transmission quality of the medium has to be measured as a function of frequency during initialisation of the system to be able to execute the bit allocation procedure: the process wherein each carrier is assigned a number of bits depending on the transmission quality of this carrier. In such systems, the information required to perform the method according to the present invention is available once the system is in operation so that no additional measurements are required to determine the shares of phase errors of different pilot carriers in the clock timing error used for synchronisation. Only the phase errors of the different pilot carriers have to be detected during the tracking mode or normal operation process.
Another advantageous feature of the method is where the clock timing error is calculated as a weighted sum of the phase errors whereby the share of the phase error equals a weight coefficient in the sum.
In this way, the clock timing error becomes a linear combination of the phase errors detected for the different pilot carriers so that calculation of the clock timing error involves low mathematical complexity. Via the weights of the different terms in the linear combination, the phase errors get different shares in the clock timing error. These weights, according to the present invention, are dependent on the transmission quality of the respective pilot carriers.
Also an advantageous feature of the present invention is such a method where the share is linearly proportional to the value of the transmission quality parameter.
As will be proven later on in this document, a maximum likelihood based approach of the problem of calculating the clock timing error out of phase errors detected for a plurality of pilot tones results in a linear relationship between the weights and the transmission quality of the pilot carriers.
Yet another advantageous feature of the present method is where a proportionality factor between the share and the value of the transmission quality parameter is linearly dependent on a frequency of the pilot carrier.
Another result of the maximum likelihood approach set out later on in this document is that, for a particular pilot carrier, the coefficient which has to be multiplied with the transmission quality value to obtain the weight related to that pilot carrier is proportional to the frequency of that pilot carrier or the pilot carrier index in case the frequency is determined thereby.
A further feature of the method according to the present invention is where the weighted sum is normalised by a linear combination of values of the transmission quality parameter measured for the plurality of pilot carriers.
In this way, the weights are normalised.
Still a feature of the present invention is where a coefficient in the linear combination depends on a square frequency of a pilot carrier of the pilot carriers.
Thus, the complete gain of the arrangement that determines the clock timing error is made equal to one. This feature is particularly advantageous in a system where the number of pilot carriers used for synchronisation is adaptive. Independence of the level of the clock timing error from the number of pilot carriers used to determine this clock timing error, obtained by such a normalisation is advantageous from the point of view of hardware implementation.
Moreover, a feature of the present invention is where the transmission quality parameter is a signal-to-noise ratio.
Indeed, signal-to-noise ratio values of the different pilot carriers are excellent measures for the transmission quality of the medium between the multi-carrier modems. In an Asymmetric Digital Subscriber Line (ADSL) system operating according to the ANSI Standard T1.413-1995 entitled xe2x80x98Network and Customer Installation Interfacesxe2x80x94Asymmetric Digital Subscriber Line (ADSL) Metallic Interfacexe2x80x99, a signal-to-noise ratio value is measured for each carrier during initialisation and used for bit allocation. This is indicated in paragraphs 12.6.6, 12.7.8 and 6.5 of the cited ANSI Standard. Alternative implementations of the present invention however may use other transmission quality parameters, for instance the noise level, to determine the shares of the phase errors of different pilot carriers in the clock timing error used for synchronisation.